The functional principle of successive approximation represents a potentially useful operating method for analog/digital converters. In this method, a digital value is determined from an analog input voltage using an iterative search process. For this purpose, the analog input voltage to be converted is compared with an analog threshold voltage by means of a comparator. The threshold voltage itself is generated by the digital/analog converter. The digital input word of the digital/analog converter is generated by a successive approximation register (SAR), which is connected so as to follow the comparator. The digital input word is successively approximated to the required digital value by systematically varying the digital input word in accordance with the signal at the comparator output.
A binary network is often used to implement the digital/analog converter such that a different weight is allocated to each bit of the input word. The weight is set in accordance with the respective bit value. The weights of such a binary network are binary-weighted, i.e. their weight values are related to one another in powers of two. Such binary networks are typically implemented by means of binary-weighted current sources, an R-2R resistance network, or binary weighted capacitors.
Instead of the binary search strategy described above, non-binary approaches (for example with a base of 1.85 instead of 2) have also been conceived that provide digital/analog converters with redundancy. Although such systems require more clock cycles due to the redundancy, the systems can be clocked more rapidly so that such a non-binary search strategy can still produce results more rapidly.
However, compared with binary networks, non-binary networks have the potential disadvantage that the individual weights, in principle, exhibit relatively poor matching. Moreover, any self-calibration of the weights in a non-binary network may be relatively complex.